Poll Error -Finding the Right Context (2.8VSE)
Use API Methods



Similarly to https://blender.stackexchange.com/a/101129/15543 ditch the operator completely here and use API methods.  Sequences.new_effect()

import bpy

context = bpy.context
scene = context.scene
sed = scene.sequence_editor
sequences = sed.sequences
active_strip = sed.active_strip
if active_strip:

    color_strip = sequences.new_effect("Color", 'COLOR', 
           active_strip.channel - 1,
           frame_start=active_strip.frame_start, 
           frame_end=active_strip.frame_final_end)

    color_strip.color = (1, 0, 0)

    cross_strip = sequences.new_effect("Cross", 'CROSS', 
           active_strip.channel + 1,
           frame_start=active_strip.frame_start, 
           frame_end=active_strip.frame_final_end, 
           seq1=active_strip,
           seq2=color_strip)
    cross_strip.blend_type = 'CROSS'
    cross_strip.blend_alpha = 0.5